1. Field of the Invention
The present invention relates to a low power semiconductor device.
2. Description of the Prior Art
In a conventional semiconductor device, circuit constants are set in such a way that the semiconductor device can drive the heaviest load (in other words, the load having the lowest resistance) within the range of loads for which it is designed to drive. For example, as FIG. 3 shows, in the case of an output-stage transistor 3 for driving an external load 100, the current IB that is fed to the base of the transistor 3 to turn it on is set in such a way that the collector current IC of that transistor 3 fulfills
IC=(VCCxe2x88x92VLOW)/RLMIN
where VCC represents the supply voltage, VLOW represents the voltage desired at the output terminal OUT when the output-stage transistor 3 is turned on, and RLMIN represents the minimum value of the resistance of the load that can be connected as the load 100. Once set, this current IB that is fed to the base of the output-stage transistor 3 to turn it on is kept fixed.
Thus, in a conventional semiconductor device, the lighter the load (in other words, the higher the resistance of the load), the larger the amount of extra current that is fed to the base of the output-stage transistor when it is turned on, and thus the higher unnecessary current consumption. To reduce this unnecessary current consumption, it is necessary to narrow the range of resistance of loads for which the semiconductor device is designed to drive.
Moreover, since the output-stage transistor is turned on with a fixed base current irrespective of the resistance of the load, the output voltage obtained when the output-stage transistor is turned on varies greatly according to the resistance of the load.
An object of the present invention is to provide a semiconductor device that operates with reduced unnecessary current consumption, and with reduced variation in the output voltage resulting from variation in the resistance of the load, but without narrowing the range of resistance of loads for which the semiconductor device is designed to drive.
To achieve the above object, according to one aspect of the present invention, a semiconductor device is provided with a transistor that controls, according to the resistance of the load externally connected to the output terminal thereof, the current fed to the base of an output-stage transistor for driving the load to turn it on.
In this circuit configuration, even if circuit constants are so determined as to permit the semiconductor device to drive the heaviest permissible load, only a reduced amount of extra current is fed to the base of the output-stage transistor when it is turned on with a light load connected to the semiconductor device. In this way, it is possible to reduce unnecessary current consumption, without narrowing the range of resistance of loads for which the semiconductor device is designed to drive. Moreover, the lighter the load, the lower the current that is fed to the base of the output-stage transistor to turn it on. This helps reduce variation in the output voltage resulting from variation in the resistance of the load.
According to another aspect of the present invention, a semiconductor device is provided with: a constant-current source; a current amplifier circuit that is connected to the output side of the constant-current source; a first transistor that receives at the base thereof the current output from the current amplifier circuit, has the emitter thereof connected to a reference potential, and has the collector thereof connected to an output terminal; and a second transistor, of the opposite conductivity type to the first transistor, that has the base thereof connected to the output terminal, has the emitter thereof connected to the output side of the constant-current source, and has the collector thereof connected to the reference potential.
In this circuit configuration, the second transistor operates in such a way that, the lighter the load, the lower the input current to the current amplifier circuit. As a result, only a reduced amount of extra current is fed to the base of the first transistor when it is turned on. In this way, it is possible to reduce unnecessary current consumption without narrowing the range of resistance of loads for which the semiconductor device is designed to drive. Moreover, the lighter the load, the lower the current that is fed to the base of the output-stage transistor to turn it on. This helps reduce variation in the output voltage resulting from variation in the resistance of the load.
In this semiconductor device, it is also possible to additionally provide a third transistor that has the base thereof connected to the output side of the constant-current source, has the emitter thereof connected to the reference potential, and has the collector thereof connected to the output terminal, with the second transistor parasitic on the third transistor. This helps increase the withstand voltage against electrostatic destruction. Alternatively, it is also possible to additionally provide a supply voltage detection circuit that chooses whether to short-circuit the output side of the constant-current source to the reference potential or not according to the supply voltage.